1. Field of the Invention
The present invention relates to a photoelectric conversion device using a semiconductor substrate, and more particularly to a solar battery. The structure of the solar battery is applicable to the solar batteries of various types using a bulk semiconductor such as a known mono-crystal wafer or polycrystal wafer.
Also, the present invention relates to a photoelectric conversion device using a thin film semiconductor formed on an insulating or conductive substrate for a photoelectric conversion layer, and the solar battery using the thin film semiconductor is applicable to the solar batteries of various types.
The present invention is applicable to a junction type solar battery based on a p-n junction, a non-junction type solar battery having a Schottky barrier, a MIS structure or the like, a multi-layer junction type solar battery, a hetero junction type solar battery and so on.
2. Description of the Related Art
The solar batteries can be manufactured using a variety of semiconductor material or organic compound material, however, from the industrial viewpoint, silicon which is semiconductor is mainly used for the solar batteries. The solar batteries using silicon can be roughly classified into a bulk type solar battery using a wafer of mono-crystal silicon, polycrystal silicon or the like and a thin film solar battery having a silicon film formed on a substrate.
Also, the reduction of the manufacture costs has been required for the spread of the solar batteries, and in particular, the thin film solar battery has been expected to provide the effects of the reduced costs because the raw material used for the thin film solar battery is reduced in comparison with the bulk type solar battery.
At present, in the field of the thin film solar battery, an amorphous silicon solar battery has been put to practical use. However, because the amorphous silicon solar battery is lower in conversion efficiency than the solar batteries using mono-crystal silicon or polycrystal silicon, and also suffers from problems such as light deterioration, its use is limited. For that reason, as another means, the thin film solar battery using a crystalline silicon film has also been developed.
As mentioned above, although the high conversion efficiency and the reduction of the manufacture costs are required at the same time for the solar battery, both are substantially contrary to each other. For example, in the case of pursuing the conversion efficiency, it can be relatively readily achieved by using the mono-crystal wafer of a high grade (defects and the like are remarkably reduced) although the manufacture costs are increased as much.
On the contrary, under existing circumstances, even though the costs can be reduced by using the mono-crystal wafer of a low grade (so-called solar battery grade, etc.), the conversion efficiency is slightly lowered unavoidably. In particular, the polycrystal wafer, the thin film solar battery and so on have been developed for primarily reducing the costs, so that the conversion efficiency is subordinate to the reduction of the costs.
Also, in view of the low costs, attention has been paid particularly to the thin film solar battery using the crystalline silicon thin film. However, since the crystalline silicon thin film, for example, the mono-crystal silicon thin film is small in absorption coefficient, the thin film solar battery suffers from such a problem that it does not sufficiently function as the photoelectric conversion device without thickening the film.
Since a grain boundary always exists in the polycrystal silicon thin film generally obtained, the grain boundary forms an electronic state having an energy corresponding to a forbidden band, to thereby shorten the lifetime of carriers. In other words, when the film is thickened, the carriers are recombined before they reach an electrode, thereby making it difficult to ensure a sufficient photoelectric conversion efficiency.
As described above, importance has been brought into the development of the solar battery to which attention has been paid as a new energy source in recent years, and a large problem on how to manufacture a solar battery high in conversion efficiency while restraining the manufacture costs arises toward the development of an energy in the future.
The present invention has been made to solve the above problem, and therefore an object of the present invention is to provide a technique by which a crystalline silicon substrate of a low grade (for example, a solar battery grade, etc.) frequently used in a bulk type solar battery is changed into a silicon substrate having a crystalline property of a high grade (for example, a semiconductor grade, etc.).
Then, the first object of the present invention is to provide a bulk type solar battery that can achieve a high conversion efficiency and the low costs together by manufacturing the solar battery using the crystalline silicon substrate having the high crystalline property.
A second object of the present invention is to provide a technique by which a crystalline silicon thin film that forms a photoelectric conversion region is formed of a crystalline silicon thin film which is reduced in defects and has a high crystalline property, etc., more than those of the conventional crystalline silicon thin film.
Then, the second object of the present invention is to provide a bulk type solar battery that can achieve a high conversion efficiency and the low costs together by manufacturing the solar battery using the crystalline silicon thin film having the high crystalline property.
In order to solve the above problems, according to the present invention, in a photoelectric conversion device using a semiconductor substrate having crystalline property, catalytic elements are segregated in defects inside of the semiconductor substrate. Thereafter, while the catalytic elements within the semiconductor substrate are gettered by halogen elements or the elements of group XV, the defects inside of the semiconductor substrate are removed.
The above semiconductor substrate may be a substrate using Si material, GaAs material or CdS material. More particularly, as a general example, it may be a mono-crystal silicon wafer or a polycrystal silicon wafer. At present, there have been known a variety of means for forming the solar battery on the silicon wafer.
For example, it is generally structured such that a layer having a weak reverse conductive type (an n-type layer) is formed, for example, on a surface of a p-type silicon wafer, to form a p-n junction, carriers collected on the n-type layer are drawn out to obtain a current. Also, apart from the p-n junction, there is a case in which a junction form such as a p-i-n junction (in case of crystalline silicon, it is substantially directed to a p+-p-n junction, p+-pxe2x88x92-n junction, etc.) is applicable.
Also, according to the present invention, a semiconductor substrate having a crystalline property and a layer containing catalytic elements which is in close contact with an upper surface of the semiconductor substrate are subjected to a heat treatment, and thereafter while the catalytic elements inside of the crystalline semiconductor thin film are segregated by halogen elements or the elements of group XV, the defects inside of the semiconductor thin film are removed.
In the above structure, the substrate material may be, for example, ceramic, stainless steel, metal silicon, tungsten, quartz, sapphire, etc. Also, although a variety of means for manufacturing the thin film solar battery have been reported, the present invention is applicable not only to the structure and manufacture method of the solar battery but also to wide fields.
For example, there is a method in which impurities such as P(phosphorus) which give a reverse conductive type are added to a crystalline silicon thin film having a substantially intrinsic i-type (or weak p-type) in the vicinity of a main surface thereof (a side to which a solar light is made incident), to form an i-n (or p-n) junction. Also, there is a case in which a crystalline silicon layer having an n-type is laminated on a crystalline silicon thin film to form an i-n (or p-n) junction. Further, a p-i-n junction in which a p-type conductive layer, a substantially intrinsic i-type conductive layer and an n-type conductive layer are laminated may be formed as occasions demand.
The solar battery formed in the above manner is structured such that carriers are produced by light excitation on a junction of the i-type (or p-type) layer and the n-type layer, and the carriers collected on the n-type layer are drawn out by an electrode to obtain a current. Also, there is a case in which a junction such as a p+-p junction is used other than the p-n junction and the p-i-n junction.
The basic object of the present invention is to use an action that promotes the crystallization of catalytic elements in order to improve the crystalline property of the photoelectric conversion region in the photoelectric conversion device such as the bulk type solar battery or the thin film solar battery, etc.
However, although the catalytic elements has the action that promotes the crystallization, they have a risk that the electric characteristics and the reliability are lowered when the catalytic elements are contained in the photoelectric conversion region. For that reason, in the present invention, in order to remove the catalytic elements from the photoelectric conversion region, the gettering action of halogen elements or the elements of group XV is used.
In the above-mentioned structure, the catalytic elements may be Fe, Co, Pt, Cu, Au or the like. The method of forming the catalytic element layer may be the plasma processing method, the vapor deposition method, the spin coating method and so on. Preferably, the vapor deposition method and the spin coating method are advantageous because they are difficult to damage the semiconductor substrate surface or the semiconductor thin film. In particular, the present inventors mainly apply the spin coating method because it is excellent in the controllability of the added amount of the catalytic elements.
Also, in the above-mentioned structures, the method of gettering the catalytic elements is roughly divided into two kinds of methods. A first method is a method of conducting a heat treatment in an atmosphere containing halogen elements therein. It is particularly preferable that the halogen elements available to gettering are Cl(chlorine) and F(fluorine). The gas used for adding the halogen elements to the atmosphere is one or a plurality of kinds selected from HCl, HF, HBr, Cl2, NF3, ClF3, F2 and Br2.
A second method is a method in which a layer containing elements belonging to group XV are formed in contact with a surface of a semiconductor substrate or a surface of a semiconductor thin film, and the surface of the semiconductor substrate or the surface of the semiconductor thin film, and the layer containing elements belonging to group XV are subjected to a heat treatment.
For the formation of the layer containing the elements belonging to group XV, there can be applied a liquid layer method or a gas phase method. For example, a silicon oxide film or a silicon film to which the elements belonging to group XV have been added may be formed.
Alteratively, there can be applied a method in which the elements belonging to group XV are introduced into the semiconductor substrate or the semiconductor thin film through the ion-implantation method, the plasma doping method or the like.
The elements belonging to group XV may be N(nitrogen), P (phosphorus), As(arsenic), Sb(antimony) and Bi(bismuth). In particular, P exhibits a remarkable gettering effect.
Also, it is important that the concentration of the elements belonging to group XV within the layer containing the elements belonging to group XV is set to be higher at least by one digit than the concentration of catalyst within the semiconductor substrate or the semiconductor thin film. For example, if the concentration of catalyst within the semiconductor substrate in which the catalyst was segregated is approximately 1xc3x971019/cm3, then the concentration of the elements of group XV within the layer containing the elements that belongs to group XV is set to be 1xc3x971020/cm3 or more.
Also, in gettering using the halogen elements or the elements belonging to group XV, the effect of gettering changes according to a heat temperature and a heat period. For that reason, the conditions of the heat treatment for gettering may be set appropriately considering the characteristic of a material such as a heat resistance of the substrate, the productivity, the economics or the like.
Now, a description will be given in more detail of preferred embodiments of the present invention with reference to the accompanying drawings.
A embodiment of the present invention will be described with reference to FIGS. 1A to 1D. The first embodiment of the present invention is related to a bulk type solar battery and shows an example using halogen elements to remove Ni that is used as catalytic elements.
In FIG. 1A, reference numeral 101 denotes a semiconductor substrate that is a mono-crystal silicon substrate of the solar battery grade in this example. In FIG. 1A, what are indicated by marks xe2x80x9cXxe2x80x9ds inside of the semiconductor substrate 101 are defects that slightly remain inside of the mono-crystal silicon. The defects are dislocations, non-bonds or the like, and are difficult to remove unless a heat treatment at a very high temperature of 1,100xc2x0 C. or higher is conducted.
It is needless to say that another silicon substrate having the crystalline property can be applied. Therefore, although this embodiment shows an example using a monocrystalline silicon substrate, for example, a polycrystal silicon substrate may be applied likewise.
Subsequently, a layer containing Ni(nickel) elements is formed on a surface of the semiconductor substrate 101. The formation of a Ni layer 102 can be achieved by using the plasma processing method, the vapor deposition method, the spin coating method, or the like. Preferably, the vapor deposition method and the spin coating method are advantageous because they are difficult to damage the surface of the substrate. In particular, the spin coating method is excellent in controllability when a very small amount of Ni is added to the semiconductor substrate, and therefore the present inventors mainly apply this method.
After a state shown in FIG. 1A is thus obtained, a heat treatment is conducted at a temperature ranging from 450 to 700xc2x0 C. It is preferable that the heat treatment is conducted under an inactive atmosphere. The heat treatment allows Ni elements to diffuse from the Ni layer 102, to thereby produce Sixe2x80x94Ni bonds in the defects indicated by the marks xe2x80x9cXxe2x80x9ds by priority. The reason why the defects indicated by the marks xe2x80x9cXxe2x80x9ds in FIG. 1A are indicated by marks xe2x80x9cOxe2x80x9ds in FIG. 1B is to represent such a fact that the Ni elements are bonded to the defects so that the defects are substantially compensated.
Through a heat treatment shown in FIG. 1B, the Ni layer 102 reacts with the semiconductor substrate 101 so that a nickel silicide layer 103 is formed on the surface of the semiconductor substrate 101. It is desirable that the nickel silicide layer 103 is previously etched and removed with chemicals such as hydrofluoric acid prior to an Ni gettering process which will be conducted succeedingly, because the nickel silicide layer 103 contains Ni elements at a high concentration.
Subsequently, the Ni elements diffused inside of the semiconductor substrate 101 are removed by gettering. The elements applicable for gettering are preferably halogen elements such as Cl or F. A gas for introducing the halogen elements is one or a plurality of kinds selected from HCl, HF, HBr, Cl2, NF3, F2 and Br2.
This embodiment shows an example in which the gettering process is conducted through the gas phase method using a gas containing the above halogen gas as its component, representatively a gas such as HCl or NF3. This process is conducted at a temperature ranging from 700 to 1100xc2x0 C. and in a state where the gas such as HCl or NF3 is contained in the atmosphere where the heat treatment was conducted.
In a state thus obtained as shown in FIG. 1C, most of the Ni elements that existed inside of the semiconductor substrate 101 are gettered and removed by the halogen elements so that the Ni elements are taken in an oxide film 104 formed on the surface of the semiconductor substrate 101, or are changed into a gas phase and made volatile. Accordingly, no Ni element substantially exists inside of the semiconductor substrate 101, and even though the Ni elements exist therein, its concentration is reduced to 1/10 to 1/1000. Specifically, it can be reduced to 5xc3x971018/cm3 or less, preferably 5xc3x971016/cm3 or less. The concentration of the elements in this specification is defined as a minimum value of a measured value which was measured by SIMS (secondary ion mass spectrometry).
Although the semiconductor substrate 101 contains another metal elements to some degree from the beginning because of the solar battery grade, it is presumed that the impurities inside of the semiconductor substrate 101 are almost removed since those metal elements are also gettered together with the removal of the Ni elements.
Also, it is presumed that during the heat treatment, the following reaction occurs inside of the semiconductor substrate 101. First, it is presumed that when the Ni elements are gettered and eliminated by the halogen elements, the following reaction occurs representatively.
2Sixe2x80x94Ni+4Cl(or 4F)xe2x86x92Sixe2x80x94Si+2NiCl2(or 2NiF2)
As represented by the above expression, since non-bonds of Si formed by the elimination of Ni are adjacent to each other, they are recombined with each other with an energy less than that of the no-bonds indicated by the marks Xs in FIG. 1A to form Sixe2x80x94Si bonds.
In particular, because NiCl2 is high in volatility, most of NiCl2 is diffused in the gas phase except that a part thereof is taken in a thermally oxidized film during the heat treatment. FIG. 20 is a graph showing a relation between the vapor pressure and the temperature of NiCl2 (nickel chloride) as a reference material. For example, the vapor pressure of NiCl2 at 950xc2x0 C. is approximately 0.8 atm, and thus represents a pressure close to the atmospheric pressure.
As described above, in the process shown in FIG. 1C, the gettering of Ni elements and recombination of Si with each other are conducted at the same time, thereby being capable of almost eliminating the defects that exist inside of the semiconductor substrate 101. Therefore, as shown in FIG. 1D, a semiconductor substrate 105 obtained by etching and removing the oxide film 104 is reduced in the concentration of defects in comparison with the semiconductor substrate 101 of the state shown in FIG. 1A, and thus can be regarded as a silicon substrate high in grade which is equal to the silicon substrate of the semiconductor grade.
In the gettering process, the halogen elements exist inside of the semiconductor substrate 101 with the concentration of 1xc3x971016 to 1xc3x971020/cm3. There is many cases in which the halogen elements exist in such a manner that they terminate non-bonds of Si.
Also, this embodiment shows an example in which Ni elements are used as catalytic elements and gettered by halogen elements, however, the same effect can be obtained by using Fe, Co, Pt, Cu, Au or the like as catalytic elements.
As described above, the silicon substrate of the low grade such as the solar battery grade can be changed into the silicon substrate having a high crystalline property which is equal to the silicon substrate of the high grade such as the semiconductor grade in accordance with the present invention. Therefore, a solar battery that achieves a high conversion efficiency can be manufactured using a material as cheap as possible.
After the semiconductor substrate 105 shown in FIG. 1D has been obtained, impurity ions having a conductive type reverse to that of the semiconductor substrate 105 are implanted on an upper surface of the semiconductor substrate 105 (FIG. 1D). Hereinafter, a description will be given assuming that the semiconductor substrate 105 is a p-type silicon substrate.
The impurity ions (P(phosphorus) in this example) are diffused by a heat treatment to form a shallow n-type layer on a surface layer of the semiconductor substrate 105. Since the uppermost surface layer of the n-type layer is damaged by ion implantation, it is desirable that the surface layer is removed by etching or the like through the wet method. Also, in this situation, if a texture structure in which the surface of the n-type layer is intentionally formed uneven is provided, a solar light can be efficiently utilized. Also, as occasions demand, a reflection preventing film may be formed on the texture structure.
Finally, a take-out electrode is formed on each of the front surface and the rear surface of the semiconductor substrate 105 to complete a solar battery. A take-out electrode on a side to which the solar light is made incident needs to be so shaped as not to block the solar battery (for example, sinking comb-shaped). Also, a transparent electrode can be used as the take-out electrode.
The solar battery thus manufactured is formed using the cheap silicon substrate, thereby being capable of restraining the manufacture costs. Furthermore, since the silicon substrate has a crystalline property which is equal to the high grade although it is cheap and of the low grade, the bulk type solar battery that achieves a high conversion efficiency can be manufactured by such a silicon substrates.
Also, in this embodiment, although the method of improving the crystalline property of the semiconductor substrate in the bulk type solar battery was described, if the same process is conducted on the semiconductor thin film of the thin film solar battery, the thin film solar battery that achieves a high conversion efficiency can be achieved.
Another embodiment will be described with reference to FIGS. 3A to 3C. The embodiment of the present invention is related to a bulk type solar battery and shows an example using the elements of group XV to remove Ni that is used as catalytic elements.
In FIG. 3A, catalytic elements (for example, Ni elements) are first held on a surface of a semiconductor substrate 201, and then subjected to a heat treatment to diffuse the Ni elements in the semiconductor substrate 201. Then, the Ni elements are segregated to defects inside of the semiconductor substrate 201 by priority.
Subsequently, a layer 202 containing phosphorus is formed on the surface of the semiconductor substrate 201 through the gas phase method or the liquid layer method. The layer 202 containing phosphorus may be formed by phosphorus silicate glass (PSG) or phosphorus doped polycrystal silicon. Alternatively, phosphorus is implanted in the semiconductor substrate 201 in a shallow manner, thereby being capable of forming an n-type layer.
Then, as shown in FIG. 3B, the semiconductor substrate 201 and the layer 202 containing phosphorus are subjected to a heat treatment. In the case where nickel is used as the catalytic elements, and phosphorus is used as the gettering elements, heating at about 600xc2x0 C. makes nickel and phosphorus in a stable combination state, that is, Ni3P, Ni5P2, Ni2P, Ni3P2, Ni2P3 and NiP3.
For that reason, nickel in the semiconductor substrate 201 is absorbed by the layer 202 containing phosphorus through the heat treatment shown in FIG. 3B.
In the grain boundary of the semiconductor substrate 201, the unpaired bonds of silicon are combined with nickel to provide silicide in a bond state such as Sixe2x80x94Nixe2x80x94Si. However, the combination of nickel and silicon is separated by a heat treatment for gettering. Then, since non-bonds of Si formed with the separation of Ni from silicon are adjacent to each other, they are recombined with each other with an energy smaller than that of the non-bonds to form a Sixe2x80x94Si bond.
As described above, in the process shown in FIG. 3B, the gettering of Ni elements and the combination of Si and Si are conducted simultaneously, thereby being capable of almost eliminating the defects that exist inside of the semiconductor substrate 201. Hence, as shown in FIG. 3C, the semiconductor substrate 203 obtained by removing the layer 202 containing phosphorus through etching is smaller in the density of defects than the semiconductor substrate 201 of the state shown in FIG. 3A so that it can be formed as the silicon substrate high in grade which is equivalent to that of the silicon substrate of the semiconductor grade.
It should be noted that the heat treatment for gettering is conducted at 500xc2x0 C. or higher, preferably, at 550 to 650xc2x0 C. for 2 hours or longer, preferably, 4 to 12 hours in an electric furnace. The heat treatment atmosphere may be an inactive atmosphere, a hydrogen atmosphere or an oxidizing atmosphere. In particular, with the addition of halogen elements to the oxidizing atmosphere, the effects of gettering can be enhanced.
The semiconductor substrate 203 obtained in the above manner improves in crystalline property in comparison with the original semiconductor substrate. For example, the silicon substrate of the low grade can be changed into a substrate having crystalline property equivalent to that of the silicon substrate of the high grade.
In this embodiment, a method for improving the crystalline property of the semiconductor substrate of the bulk type solar batter was described. If the same processing is conducted on a semiconductor thin film of the thin film solar battery, a thin film solar battery that achieves a high conversion efficiency can be manufactured.